Radiation curable ink compositions are generally known in the art. An ultraviolet radiation ink composition comprises at least one radiation curable component and may comprise a photo initiator, a colorant and an inhibitor. Inkjet printing devices of radiation curable inks usually require a low viscosity ink. In inkjet printing, ink drops may be formed on demand by an inkjet printhead. To ensure proper formation of ink drops the viscosity of the ink in use may be required to be no more than 100 mPa·s and commonly no more than 30 mPa·s. Typically, when the ink is ejected from the printhead, the ink has a viscosity of less than 20 mPa·s, e.g. 8-12 mPa·s at an operating temperature of the printhead.
Many inkjet printheads nowadays are heated to an elevated temperature in order to reduce the ink viscosity to a desired level. The operation temperature of the printhead is however restricted due to on one side the design and material choice of the printhead and on the other side the thermal stability of the radiation curable ink composition. Generally, in the prior art, inkjet printheads for radiation curable inks are operated around 45-55° C.
Ink components, especially the curable component and/or the photoinitiator, are selected in order to provide proper curing properties. The curing speed and curing quality depends furthermore on the ultraviolet radiation means used in the application. In curing quality a distinction may be made between outer surface curing level and inner curing level of the ink layer on top of the recording medium. The adhesion of the ink layer to the recording medium may also be effected by the curing quality.
In the application of ink on rigid recording media, the adhesion of the ink layer onto the recording medium is known to be an important property. Certain rigid recording media are relatively critical towards adhesion of radiation curable inks. For instance recording media having relatively apolar surface properties, such as polystyrene, tend to show less adhesion towards radiation curable inks. An important aspect of radiation curable ink components is their reactivity under curing conditions. A known drawback of many of these reactive components is their health and safety risks; for example reactive components may have (skin) irritancy properties and/or provide sensibilisation effects on the longer term. Health and safety risks of (uncured) inks are indicated by standardised labels.
The health and safety risks depend inter alia on the specific components selected, the amount of component used in the ink and the combination with other components in the ink composition. In order to reduce the health and safety risks the ink during application is cured. In general, after curing, the ink composition in image applications looses at least a part of the health and safety risks, in particular the risks which are related with the radiation curable nature of the ink composition, thereby limiting the health and safety risks of the resulting print. However, prior art uncured inks are known to exhibit such safety and health risks when not cured, while such uncured inks may be handled by unsuspecting users, e.g. for supplying ink to a printing device.